Simulation
Simulation involves the assessor wearing physical simulators to mimic the effects of functional impairments. The Third Age Suit used by Ford is a very sophisticated example of this approach, but even simple simulations can be very effective. It is possible to buy goggles designed to reproduce a wide range of visual impairments from glaucoma to tunnel vision. Hand movements can be restricted by binding them or by using gloves with stiffeners in the fingers.
If the simulators are calibrated appropriately, they can even form the basis of an absolute, quantitative scale for measuring the inclusivity of the product (vision impairment level 6, glove stiffness level 4, etc.).
Simulation is a very popular concept with designers. It offers the benefits of self-observation (speed of assessment, no need for user recruitment) with the additional bonus that the designers can experience some of the difficulties faced by users with functional impairments.
However, it simply provides a feeling for the effects of the impairment. For example, simply closing your eyes allows you to experience the difficulties of navigating without being able to use your eyes, but it is not the same as never having experienced colours, as is the case for some congenitally blind people.
This is an important distinction and must be borne in mind by designers. Simulation will never replace entirely the need to involve users in the design and assessment process.